Cyclopentadienone iron alcohol complexes: synthesis, reactivity, and implications for the mechanism of iron-catalyzed hydrogenation of aldehydes

Abstract

Cyclopentadienone iron alcohol complexes generated from the reactions of [2,5-(SiMe(3))(2)-3,4-(CH(2))(4)(eta(5)-C(4)COH)]Fe(CO)(2)H (3) and aldehydes were characterized by (1)H NMR, (13)C NMR, and IR spectroscopy. The benzyl alcohol complex [2,5-(SiMe(3))(2)-3,4-(CH(2))(4)(eta(4)-C(4)CO)]Fe(CO)(2)(HOCH(2)C(6)H(5)) (6-H) was also characterized by X-ray crystallography. These alcohol complexes are thermally unstable and prone to dissociate the coordinated alcohols. The alcohol ligand is easily replaced by other ligands such as PhCN, pyridine, and PPh(3). Dissociation of the alcohol ligand in the presence of H(2) leads to the formation of iron hydride 3. The reduction of aldehydes by 3 was carried out in the presence of both potential intermolecular and intramolecular traps. The reaction of 3 with PhCHO in the presence of 4-methylbenzyl alcohol as a potential intermolecular trapping agent initially produced only iron complex 6-H of the newly formed benzyl alcohol. However, the reaction of 3 with 4-(HOCD(2))C(6)H(4)CHO, which possesses a potential intramolecular alcohol trapping agent, afforded two alcohol complexes, one with the newly formed alcohol coordinated to iron and the other with the trapping alcohol coordinated. The intramolecular trapping experiments support a mechanism involving concerted transfer of a proton from OH and hydride from Fe of 3 to aldehydes. The kinetics and mechanism of the hydrogenation of benzaldehyde catalyzed by 3 are presented.

Figure 1

Examples of ligand-metal bifunctional catalysts.

Figure 2

X-ray Crystal Structure of 6-H shown with 50% probability ellipsoids.

Figure 3

Rate of Ligand Substitution of Alcohol from 6-H as a Function of PhCN and PPh₃ Concentrations at 5 °C.

Figure 4

Rate of PhCN Substitution of Alcohol from 6-H as a Function of Added Alcohol Concentration at 5 °C with [PhCN] = 0.35 M.

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